As far as hiring an attorney? Don't pull that trigger until you've done a little self-education. You may find you can handle this on your own by just doing a little R&D and filing your own complaint with the E&O insurance company.
Too many folks run to attorney's and spend a lot of money before taking a little time to do a little investigating on their own. The more information and knowledge you have the more prepared and educated you will be which will result in less time any attorney will have to spend on your case.
At that point, you may want to contact the listing agent's broker with your research that has been checked out that indeed, the permits were not present on the additions. At that point, I would think the broker will direct you to their errors & omissions insurance carrier to open up a claim. Best of luck, Caroline